Some communication systems employ one or more network nodes that are only available for communication over a specific network during a part of a communication period. Such network nodes thus switch between an active part of the communication period wherein the network node is active to communicate over the specific network and an inactive period of the communication period wherein the network node is not available for communication over the specific network. For example, a low-power network node may be active with a duty cycle of the communication period such that the network node can be switched between a full power, full functional communication mode during the active part of the duty cycle and a low power mode during the inactive part. When another node wants to send data to such low-power network node over the specific network, the other node may find that data is not always received by the low-power network node, and the other node may be required to resend the data until the other node sends the data while the low-power network node is in the active part of the communication period. As another example, a multiple-network node, such as a dual-network node, may be available during a first part of a communication period to communicate exclusively over a first specific network, and during a second part of the communication period to communicate exclusively over a second specific network. Also in this other example, data send by another node over the first specific network can only be received while the multiple-network node is active and data needs to be resend of the multiple-network node is not available to communicate over the first specific network.
Methods are known wherein network nodes try to send data and resend data if it is detected that the data is not received. This results in many retries and a high over-the-air traffic including many failed transmissions. Other prior art methods are known to synchronize networks using special control packets, referred to as beacon packets. As an example of such prior art methods, U.S. Pat. No. 8,254,290 B2 describes a method and apparatus for using a synchronous sensor network medium access control (MAC) protocol such as a ZigBee or IEEE 802.15.4 low-rate wireless personal area network (WPAN). The beacon packets are sent periodically and result in itself in additional over-the-air traffic compared on top of the over-the-air traffic of the data packets. Further, the additional over-the-air traffic may lead to more retries when trying to send data over the network, as retries may be needed when the network is not clear. Also, exchanging these additional beacon packets may decrease the battery life of the nodes.